docs: Render single-line code blocks

What does this MR do?

docs: Render single-line code blocks

This is a workaround for #2102 (closed), whereby single-line code blocks render as one or more [object Object] instead of the actual syntax highlighted code block on initial page load.

Addresses #2102 (closed).

This is the Fish shell command I used to apply this change:

for md in src/**/*.md
    sed -zi 's|```html\n\([^\n]*\)\n```|<!-- Empty initial line is a workaround for https://gitlab.com/gitlab-org/gitlab-ui/-/issues/2102 -->\n```html\n\n\1\n```|g' $md
end

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